1. Field of the Invention
The invention generally relates to X-ray film viewers, and more specifically to such a device with automatic light intensity compensation for changes in load and line voltage and aging of the source of illumination.
2. Description of the Prior Art
In many applications, transparencies or negatives are not used to print positive pictures, but are viewed directly by transmission of light through them. One very common such application is the viewing of X-rays by doctors and other medical personnel. Industrial X-ray pictures are also viewed in this manner to examine mechanical parts and the like and illuminated viewing boxes are used in photography to examine negatives for selection for printing of pictures.
Typically, X-rays and other transparencies are viewed by placing same against a light transmissive screen, such as a light diffusion plate, which forms one wall of a film viewer which houses a source of illumination such as incandescent, halogen, or fluorescent light sources. A detailed observation of the X-rays is facilitated by transmission through the film of adequate intensity light. However, the brightness of the light source cannot be excessive since that would make extended examinations of the films difficult and the viewer would be exposed to substantial levels of light intensity when the film is removed. The viewing of a large number of such negatives in succession would require that the viewer's eyes continuously adjust between very high intensity light levels and significantly lower light intensity levels, and this could make the viewing uncomfortable and tedious.
U.S. Pat. No. 4,118,654, issued on Oct. 3, 1978 to Ohta et al. discloses an automatic light intensity control for an X-ray film viewer. Ohta et al. teach the use of a photo-detector mounted externally of the film viewer so that it receives the same light that passes through the X-ray film which is viewed by the observer. The photo-detector is intended to only detect the intensity of light which penetrates through the film. While Ohta et al. intend to shield the photosensor from ambient light and only respond to light which passes through the film, it is possible that ambient light will modify the results since the photo-detector is mounted outside of the X-ray film viewer.
Ohta et al. also teach the use of a control knob for manually setting the intensity level of the light. Therefore, the user can presumably adjust the light intensity output level at or near the maximum or nominal light level intensity, or at a level substantially below the nominal value. Although Ohta et al. describe a circuit for automatically controlling the light source to maintain a constant level of intensity, it is improbable that the circuit can fully compensate for line and load variations and aging of the sources of light under all circumstances, particularly when the user has already manually adjusted the units for maximum light intensity. Ohta et al. are primarily concerned with maintaining a constant light level at the output of the X-ray film, irrespective of the transmissivity of such film. The patent is not directed to the solution of the problem of maintaining the output of the source of illumination at a substantially constant level prior to passage through the X-ray film as a result of line and load variations and aging of the light source.
Another X-ray viewer provides two levels of illumination. The first level provides a lower standard intensity of illumination and is intended for preliminary viewing and general use. A second high brightness level of illumination is provided which is approximately 50% higher than the first level, and is intended for viewing of darker films or those that are less light transmissive. However, the device does not appear to provide automatic light intensity compensation, but only provides one of two selected levels of such light intensity.